I was thinking about breaking it down to just make a fun sandbox VM with some APIs for network, local storage and interacting with the user. No document format

I was thinking about breaking it down to just make a fun sandbox VM with some APIs for network, local storage and interacting with the user. No document format or anything, you get a screen to draw on and get events. And then I thought: "hmmm, everybody's going to be disappointed that the VM isn't for their pet language", so I came up with the idea of just using QEMU. Literally just give every site a bare machine it can load any image on. Make a virtual IO device for the system services (like exposing the path and query part of the URL, clipboard and linking to other machines). UI, storage and network get normal VirtIO devices. Let's keep HTTP for metadata and cache control (don't want to download big images unnecessarily), with a bunch of headers for negotiating preferred CPU architecture and other hardware stuff. It's different enough from the web that it might actually work, for some value of "working".